1756-L71 vs 1756-L73 vs 1756-L75

Overview

The ControlLogix 5570 family of Programmable Automation Controllers (PACs) from Allen-Bradley (Rockwell Automation) remains an industry-standard workhorse for medium to large-scale industrial automation. Positioned chronologically between the legacy 5560 (L6x) series and the high-performance 5580 (L8x) gigabit controllers, the ControlLogix L7 series (specifically the 1756-L71, 1756-L73, and 1756-L75) offers robust, reliable, and deterministic execution for discrete, motion, process, and safety applications.

Key to the popularity of the 5570 controller series is its battery-less operation, using a capacitor-based Energy Storage Module (ESM) to write volatile memory to nonvolatile secure digital (SD) storage during a power failure.

While the 1756-L71, 1756-L73, and 1756-L75 share the same processing architecture, backplane compatibility, and engineering software environment, they differ substantially in user memory, motion axis limits, and optimal system application scaling. Selecting the correct model avoids the costly pitfalls of either running out of PLC memory mid-project or over-specifying a CPU where a lower-tier, more cost-effective option would easily suffice.

---

Key Differences at a Glance

The core distinction between the 1756-L71, 1756-L73, and 1756-L75 lies in memory allocation. Because ControlLogix architecture does not permit memory expansion beyond the factory-purchased specification, selecting the right controller at the project's outset is critical.

• 1756-L71: Built for smaller standalone applications, localized machine control, or small conveyor segments. It contains 2 MB of user memory and is constrained to 8 axes of integrated motion over EtherNet/IP.
• 1756-L73: The undisputed standard model for typical manufacturing lines, complex skids, and system integrations. It is equipped with 8 MB of user memory and can control up to 100 axes of integrated motion.
• 1756-L75: Designed for large scale process systems, DCS integrations, complex plant-wide coordination, or memory-heavy recipes. It packs 32 MB of user memory and supports up to 100 axes of integrated motion.

---

Specifications Comparison

This table details the physical, electric, environmental, and logic capacities of each processor.

---

Performance & Capabilities

Internally, the L71, L73, and L75 share an identical high-performance execution engine. They operate on a dual-core central processing unit. One core is dedicated exclusively to processing the application logic, while the second core handles backplane communication and overhead processing. This prevents communication traffic from interrupting logic execution times.

Scan Time and Execution Speed

The execution of basic boolean instructions in the 1756-L7x controllers can occur as fast as 12 nanoseconds. However, as memory pools fill, execution times are affected by the sheer volume of tags and routines. The 1756-L75 can host vastly more complex code, meaning its average loop execution time can end up longer in practice simply due to processing a 32 MB application compared to the L71's maximum 2 MB limit.

Integrated Motion Capabilities

While execution times are similar, there is a distinct tiered limit on Integrated Motion over EtherNet/IP:

• The 1756-L71 is hard-capped within the programming software to allow a maximum of 8 virtual or physical axes of coordinated motion.
• The 1756-L73 and 1756-L75 unlock the full processing capability of the Logix platform, permitting up to 100 axes of multi-drive coordination.

Battery-Free Backup (ESM)

A breakthrough over the older 1756-L6x controllers is the elimination of lithium batteries. The 1756-L7x series utilizes an Energy Storage Module (ESM) containing ultra-capacitors. When the chassis loses power, the ESM delivers enough localized electrical reserve to allow the processor to write its volatile system memory directly into the nonvolatile 1784-SD1 secure digital card.

This means you do not have to schedule annual battery maintenance or worry about program loss during long-term line shutdowns. Standard units ship with the 1756-ESMCAP module, though specialized units can be retrofitted with the 1756-ESMNSE (no-sodium ESM required for specific flame-proof or mining environments) or the 1756-ESMNK (conformal coated version).

---

Programming & Software

All three controllers are programmed using Rockwell Automation’s Studio 5000 Logix Designer (formerly known as RSLogix 5000).

Firmware & Software Version Compatibility

• Minimum software version: Studio 5000 / RSLogix 5000 Version 20.
• Maximum software version: Fully compatible through the latest Studio 5000 releases (Version 36+).

Unlike some competitor CPUs that require different IDE platforms depending on the processor scale, the L71, L73, and L75 share exact system parity. A system built on a 1756-L71 can be instantly exported, upgraded, and downloaded into a 1756-L75 within minutes with no code rewriting required—only a controller-type change in the software's Controller Properties window.

Programming Features

The controllers fully support standard IEC 61131-3 languages:

• Ladder Diagram (LD)
• Structured Text (ST)
• Function Block Diagram (FBD)
• Sequential Function Chart (SFC)

Additionally, they support User-Defined Data Types (UDTs) and Add-On Instructions (AOIs). These custom instruction blocks are memory-intensive. When developing heavy libraries of complex AOIs, the 1756-L71’s 2 MB limit is reached quickly. In contrast, the 1756-L75 provides virtually limitless capacity for extensive standard programming templates and diagnostics.

---

Communication & Networking

It is important to emphasize that none of these three controllers have an onboard Ethernet port for plant integration. The physical USB port located on the front cover is exclusively designed for local programming, firmware flashing, and configuration.

To connect an L71, L73, or L75 to an industrial network, you must install a separate, dedicated communications module into the 1756 chassis.

Primary Networking Modules

• EtherNet/IP: 1756-EN2T (single port), 1756-EN2TR, or 1756-EN3TR (dual port for Device Level Ring topologies).
• ControlNet: 1756-CN2 or 1756-CN2R.
• DeviceNet: 1756-DNB.

Historically, the connection limits for the L7 series are governed by the communication module rather than the processor itself, with the L7 processors supporting a maximum of 500 CIP (Common Industrial Protocol) connections across the backplane.

---

Pricing & Lifecycle

The 1756-L7x family represents classical industrial technology. The series is currently classified by Rockwell Automation as Active Mature. This status indicates that while these models are actively manufactured and fully supported in terms of technology and warranties, they are not recommended for new greenfield facility designs where the newer L8x platform would prove more beneficial.

Cost vs. Value Dynamics

In terms of purchase cost, Rockwell’s list prices scale progressively with memory size:

• 1756-L71: The lowest-priced model, offering an entry-point solution.
• 1756-L73: Offers the most balanced ratio of price-to-performance for general industrial projects.
• 1756-L75: Has a higher price point reflecting its large 32 MB user memory capacity.

Because these parts are frequently used in active processing lines, their value is heavily driven by availability. For maintaining existing lines, utilizing reputable industrial parts distributors like Palm Parts Solution ensures access to reliable, certified surplus or remanufactured L7 units to prevent costly downtime.

---

When to Choose Each

Choose the 1756-L71:

1. Isolated Assembly Workstations: When controlling standard indexing stations, localized test benches, or simple conveyors.
2. Strict Budget OEM builds: Standard machinery with limited dynamic variable tags, localized HMI screens, and very low (or zero) motion requirements.
3. Basic Gateway Applications: For mapping a limited block of data points from legacy devices over DH+ to EtherNet/IP via a chassis bridge.

Choose the 1756-L73:

1. Standard Manufacturing Lines: The standard option for complex assembly conveyors, packaging machines, or multi-axis palletizers.
2. Moderate Axis Motion: Designed to drive up to 100 integrated motion axes (typically keeping within 10 to 30 axes for peak scan optimization).
3. SCADA Node Integrations: For machines that require clean, moderate interfaces with plant-wide SCADA databases and multiple HMIs.

Choose the 1756-L75:

1. Data-Heavy Process plants: Where high-density analog loops, extensive P&ID control loops, and batch processing profiles require large data tables.
2. Extensive Recipe Storage: If the controller must process major dynamic operational databases locally on the CPU instead of relying on external software.
3. Heavy AOI Usage: If your plant standardization relies heavily on nested, memory-intensive diagnostics, custom AOIs, and UDTs.

---

Migration & Upgrade Path

Upgrading from Legacy L6x (ControlLogix 5560) to L7x

For older systems using a 1756-L61, 1756-L62, or 1756-L63, upgrading to an L7 series controller is a seamless swap. This upgrade offers several key benefits:

1. Drop-In Chassis Compatibility: The L7x fits directly into existing 1756 rack frames (e.g., 1756-A4, -A7, -A10, -A17 series B racks) and uses existing power supplies (e.g., 1756-PA72, -PB72).
2. Eliminate System Batteries: Replacing the battery maintenance requirement with the ultra-capacitor 1756-ESMCAP.
3. Faster Processing Speeds: The dual-core L7 processor executes instructions and handles communication far faster than the older L6 series CPUs.

Path to the L8x (ControlLogix 5580)

If you are planning for future system designs, the natural upgrade path moves to the 1756-L8x series (such as the 1756-L81E, 1756-L83E, and 1756-L85E).

The core advantages of transitioning from the L7x to the L8x include:

• Onboard Ethernet: Built-in RJ45 physical ports supporting gigabit communications, eliminating the absolute necessity of buying a 1756-EN2T or EN3TR card.
• Significantly Faster Processing: The L8 platform offers processing speeds up to 5 to 10 times faster than the L7 series.
• Upgraded Connection Capacity: Supports up to 250 EtherNet/IP nodes natively.

However, many plants decide to stick with the L7 series because it allows them to maintain stable codebase designs, avoid extensive validation testing for legacy systems, and easily swap units without modifying existing communication layouts.

---

Frequently Asked Questions

1. Can I use the USB port on my 1756-L73 or -L75 to connect to an HMI or run peer-to-peer messaging?

No. The integrated USB 2.0 port on the front of all 1756-L7x processors is exclusively designed for local diagnostic tasks, flashing firmware, and program downloading/uploading. It cannot be used as an active industrial network port. Active communication with HMIs, auxiliary SCADA nodes, or peer-to-peer setups must run through chassis-mounted network modules such as a 1756-EN2T.

2. Can I expand the program memory of my 1756-L71 if my project outgrows the 2 MB limit?

No. Logix processing memory is set at the physical silicon stage and cannot be upgraded via software licenses or expander modules. While you can insert up to a 2 GB SD card (1784-SD2), this storage acts as nonvolatile memory backup to reload program data in the event of power loss. It cannot be used as extended execution RAM. If your 1756-L71 runs out of active user memory, you must physically replace the unit with a 1756-L73 or 1756-L75.

3. Will my existing 1756-L6x project run without issues on a newer 1756-L7x unit?

Yes, in nearly all standard applications. The upgrade involves changing the controller type inside the Controller Properties configuration window inside Logix Designer. One crucial detail to verify is the firmware compatibility of your network and I/O cards; ensure that your chassis communication modules (like the older 1756-ENBT) have matching firmware revisions to bridge properly with the newer L7x processor.

4. What is the difference between the 1756-ESMCAP and the 1756-ESMNSE module shipped with these controllers?

The 1756-ESMCAP is the standard capacitor-based energy storage module that ships with all standard L7 processors. The 1756-ESMNSE is a specialized capacitor module that does not contain any sodium compounds. Some specialized facilities, such as oil refineries, petrochemical sites, or specific mining environments, require completely sodium-free components for safety compliance.

---

Related Articles

• Understanding the Conversion steps from ControlLogix L6 to L7 Platform
• A Complete Overview of Allen-Bradley Energy Storage Modules
• Selecting isolated and non-isolated Communications Modules for ControlLogix
• Logix 5570 vs Logix 5580: When to Upgrade to the L8 Family